Roll-to-roll reactor for processing flexible continuous workpiece

ABSTRACT

The present invention provides a reactor for preparing thin films of compound semiconductors for photovoltaic devices. The reactor includes a chamber that has a bottom surface that, in some locations, has protrusions that contact the bottom surface of the substrate having the compound semiconductor to provide uniform heating and cooling of the substrate. Interior walls of the chamber can also be lined with high thermal conductivity portions and low thermal conductivity portions interposed between high thermal conductivity portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/453,470 filed on Mar. 16, 2011, which is hereby incorporated by reference in its entirety herein.

BACKGROUND

1. Field of the Invention

The present invention relates to thermal reactors, and more specifically to thermal reactors for preparing thin films of Group IBIIIAVIA compound semiconductors for photovoltaic devices.

2. Description of the Related Art

Solar cells are photovoltaic (PV) devices that convert sunlight directly into electrical energy. Solar cells can be based on crystalline silicon or thin films of various semiconductor materials that are usually deposited on low-cost substrates, such as glass, plastic, or stainless steel.

Thin film based photovoltaic cells, such as amorphous silicon, cadmium telluride, copper indium diselenide or copper indium gallium diselenide based solar cells offer improved cost advantages by employing deposition techniques widely used in the thin film industry. Group IBIIIAVIA compound photovoltaic cells, including copper indium gallium diselenide (CIGS) based solar cells, have demonstrated the greatest potential for high performance, high efficiency, and low cost thin film PV products.

As illustrated in FIG. 1, a conventional Group IBIIIAVIA compound solar cell 10 can be built on a substrate 11 that can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. A contact layer 12 such as a molybdenum (Mo) film is deposited on the substrate as the back electrode of the solar cell. An absorber thin film 14 including a material in the family of Cu(In,Ga)(S,Se)₂ is formed on the conductive Mo film. The substrate 11 and the contact layer 12 form a base layer 13. Although there are other methods, Cu(In,Ga)(S,Se)₂ type compound thin films are typically formed by a two-step process where the components (components being Cu, In, Ga, Se and S) of the Cu(In,Ga)(S,Se)₂ material are first deposited onto the substrate or a contact layer formed on the substrate as an absorber precursor, and are then reacted with S and/or Se in a high temperature annealing process.

After the absorber film 14 is formed, a transparent layer 15, for example, a CdS film, a ZnO film or a CdS/ZnO film-stack, is formed on the absorber film 14. Light enters the solar cell 10 through the transparent layer 15 in the direction of the arrows 16. The preferred electrical type of the absorber film is p-type, and the preferred electrical type of the transparent layer is n-type. However, an n-type absorber and a p-type window layer can also be formed. The above described conventional device structure is called a substrate-type structure. In the substrate-type structure light enters the device from the transparent layer side as shown in FIG. 1. A so called superstrate-type structure can also be formed by depositing a transparent conductive layer on a transparent superstrate, such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga)(S,Se)₂ absorber film, and finally forming an ohmic contact to the device by a conductive layer. In the superstrate-type structure light enters the device from the transparent superstrate side.

Contrary to CIGS and amorphous silicon cells, which are fabricated on conductive substrates such as aluminum or stainless steel foils, standard silicon solar cells are not deposited or formed on a protective sheet. Such solar cells are separately manufactured, and the manufactured solar cells are electrically interconnected by a stringing or shingling process to form solar cell circuits. In the stringing or shingling process, the (+) terminal of one cell is typically electrically connected to the (−) terminal of the adjacent solar cell. Circuits may then be packaged in protective packages to form modules. Each module typically includes a plurality of strings of solar cells which are electrically connected to one another.

In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)₂ absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor, vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance the Cu/(In+Ga) molar ratio is kept at around or below 1.0. On the other hand, as the Ga/(Ga+In) molar ratio increases, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current may typically decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition.

As mentioned above, the second step or the reaction step of the two-step process involves the reaction process of the precursor stack that is formed at the first step of the two step process. The reaction process can be performed in radiation heating reactors or thermal reactors using, for example, resistance heaters. In general the thermal reactors can be atmospheric-pressure (AP) or Sub-Atmospheric (SA) thermal reactors. Such reactors are generally designed for thermal processing of thin-film solar cell materials deposited on a metallic substrate or foil.

During a thermal process, however, the metallic substrate expands as it is heated and contracts as it is cooled. Due to the inherent lack of rigidity of such structures, such expansion and contraction cause mechanical buckling of the substrate which manifests itself as bubbles, ridges and ripples. In addition, the metallic substrates may be produced with inherent distortions such as camber, scallops and quilting. Any distortion which causes the substrate to be in contact with the reactor bottom in some areas and not in others will naturally cause a different localized heating or cooling rate in the substrate and the material thereon. The areas in contact with the reactor walls will heat or cool more rapidly than those that are not in contact. For some types of processes such as the selenization of CIGS, this may cause significant variations in the properties of the synthesized solar cell absorber. Since it is not known how to enforce uniform heating or cooling by suppressing the aforementioned substrate distortions, it is best to enforce uniform heating or cooling through radiation transfer, convection and other means that do not require contact. A natural solution to these problems is to suspend the web at a distance over the bottom of the furnace.

However, in a selenization process, the excess furnace volume required to suspend the substrate over a distance of hundreds of millimeters will reduce the concentration of reactive selenium vapor in the furnace. Such reduced concentration of selenium, in turn, reduces the quality of the CIGS semiconductor produced by such a process. Further problems can also arise when a large excess space exists under the suspended substrate. For example, the process gas containing selenium vapor is entrained by viscous drag on the top and bottom surfaces of the suspended substrate. This process gas can be dragged into the end of the ramp or cool zone and then is forced outward towards the web edge. The process gas then flows upwards and subsequently distorts gas flows above the substrate where the deposited precursor is located. This creates undesirable multidirectional gas flows which can create cross-web nonuniformity in the Selenium concentration and subsequent properties of the synthesized CIGS semiconductor

Internal design of the reactors is critical for the quality of the manufactured thin films. Deposition and growth of layers forming a thin film solar cell in a roll-to-roll or in-line process is attractive for higher throughput, lower cost and better yield of such approaches. There is a need, however, to develop roll-to-roll or in-line CIGS growth techniques in which the CIGS material composition and properties are tightly controlled.

SUMMARY OF THE INVENTION

The present invention provides a roll-to-roll thermal reactor including an elongated process chamber and a roll-to-roll reaction method for forming thin films from precursor materials deposited on substrates.

In an aspect of the present invention, the elongated chamber includes a bottom wall with a reduced contact surface region so as to minimize the physical contact between the substrate and the bottom wall of the elongated chamber of the reactor as the substrate including the precursor thereon is advanced on the reduced contact surface region. The reduced contact between the substrate and the bottom wall enables uniform heat transfer to the substrate without abrupt temperature changes when the substrate is heated and cooled.

In another aspect of the present invention, each heat zone within the elongated process chamber is lined with a high thermal conductivity layer terminating with low thermal conductivity separator regions that separate one heat zone from the next heat zone.

In one aspect, the aforementioned problems are addressed by a roll-to-roll thermal reactor to heat and react a precursor material disposed over a continuous workpiece to form a solar cell absorber. In this aspect, the reactor comprises an elongated process enclosure defined by a peripheral wall including a top wall, side walls and a bottom wall, wherein the continuous workpiece enters the elongated process enclosure from an entrance opening, advances through the elongated process chamber while contacting the bottom wall, and exits from an exit opening. In this aspect, the reactor further comprises at least one reduced contact surface region formed in an inner surface of the bottom wall so that the physical contact between the workpiece and the bottom wall is reduced, wherein the reduced contact surface region includes a plurality of fixed protrusions projecting upwardly from the inner surface of the bottom wall and wherein the workpiece touches topmost ends of the plurality of fixed protrusions.

In another aspect, the aforementioned problems are addressed by a solar cell processing system comprising a continuous workpiece containing a precursor material used to form a solar cell absorber. The system further comprises a reactor having a inlet and an outlet and a central chamber disposed between the inlet and the outlet, wherein the continuous workpiece is provided into the inlet of the reactor and extends through the central chamber and exits via the outlet wherein a portion of the reactor includes a reduced contact surface where the bottom surface of the continuous workpiece is intermittently supported by the reduced contact surface. The system further comprises a heating system that heats the reactor; and a gas supply system that provides reactive gas into the reactor so that the combination of the reactive gas and the heat from the heating system transforms the precursor material on the continuous workpiece in the solar cell absorber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a thin film solar cell including a Group IBIIIAIVA compound absorber layer;

FIG. 2 is a schematic perspective view of an elongated process chamber of one embodiment of the present invention;

FIG. 3A is a schematic partial side view of a portion of an embodiment of the elongated process chamber including a reduced contact surface region;

FIG. 3B is a schematic partial side view of a portion of an embodiment of the elongated process chamber including an insert and a reduced contact surface region in the insert;

FIG. 3C is a schematic cross sectional view of the portion of the elongated process chamber including the insert and an embodiment of the protuberances;

FIG. 4A is a schematic partial side view of a portion of an embodiment of the elongated process chamber including an insert with high thermal conductivity and low thermal conductivity regions;

FIG. 4B is a schematic partial side view of a portion of an embodiment of the elongated process chamber including the insert shown in FIG. 4A and a reduced contact surface region formed in the insert;

FIG. 5A is a schematic view of an embodiment of a roll-to-roll thermal reactor of the present invention; and

FIG. 5B is an exemplary thermal profile graph of the reactor shown in FIG. 5A.

DETAILED DESCRIPTION

Embodiments of the present invention provide a roll-to-roll thermal reactor including an elongated process chamber or reactor chamber and a roll-to-roll reaction method for forming thin films from precursor materials deposited on substrates.

In one embodiment, a roll-to-roll process of the present invention may be exemplified using a continuous workpiece or a workpiece including a multilayer Group IBIIIAVIA precursor deposited on a continuous substrate using various deposition methods. This workpiece may be processed in the elongated chamber of the present invention to manufacture Group IBIIIAVIA solar cell absorbers by reacting the CIGS precursor layer comprising copper (Cu), indium (In), gallium (Ga) and at least one Group VIA material such as selenium (Se). The roll-to-roll process reactor applies a process temperature to the workpiece at a predetermined rate and according to a predetermined temperature profile to convert the precursor layer into a CIGS thin film absorber as the workpiece is advanced through the reactor. The process involves heating the CIGS precursor layer to a temperature range of 300-700° C., preferably to a range of 400-600° C., in the presence of at least one of a reactive gas containing Se and an inert gas such as nitrogen (N₂), while advancing the workpiece through the elongated process chamber and cooling the workpiece. Heat may preferably be transferred to the advancing workpiece from the elongated chamber walls which are heated. In one embodiment, the elongated chamber of the roll to roll reactor includes a bottom wall with a reduced contact surface region so as to reduce the physical contact between the substrate and the bottom wall of the elongated chamber of the reactor as the substrate, including the precursor thereon, is advanced on the reduced contact surface region. The reduced contact between the substrate and the bottom wall enables more uniform heat transfer to the substrate with less abrupt temperature changes as the substrate is heated and cooled. In another embodiment, each heat zone within the elongated process chamber is lined with a high thermal conductivity material separated by low thermal conductivity regions. For the various embodiments described below, a high thermal conductivity material may have a thermal conductivity of greater than 5 W/m-K, and a low thermal conductivity material may have a thermal conductivity of less than 2 W/m-K.

In one embodiment, the reduced contact surface region includes fixed protuberances that may make brief incidental contact with the continuous substrate to support the workpiece above the bottom wall without creating excess volume under the workpiece. In one embodiment, the fixed protuberances include, but are not limited to, widely spaced ridges, bumps, balls, nubs, bars or other small protruding features. This design supports the workpiece, reduces the time that the continuous substrate touches the bottom wall, and reduces or eliminates drastic changes in heating rate, especially in heat ramp-up and heat ramp-down zones of the elongated process chamber. Short incidental contact with these protuberances should not have a material influence on the localized heating rate, especially when these protuberances are fabricated from a low thermal conductivity material such as fused silica.

FIG. 2 shows in a partial cutaway view, an elongated chamber 100 of a roll-to-roll reactor to process a continuous workpiece 102 (workpiece hereinafter) having a front surface 103A and a back surface 103B. In this embodiment the front surface 103A of the exemplary workpiece 102 may be the top surface of a CIGS precursor layer or stack, and the back surface 103B may be a back surface of a flexible conductive substrate, such as a stainless steel substrate. There may be a contact layer (not shown), such as a molybdenum (Mo) layer, between the CIGS precursor and the flexible conductive substrate.

In one embodiment, the elongated chamber 100 includes a peripheral wall 104 defining an internal process space 106 or process space to heat, flow a process gas and to advance a continuous workpiece 102 in a process direction ‘P’ between an entrance opening 112A located at a first end 114A of the elongated chamber 100 and an exit opening 112B located at a second end 114B of the elongated chamber 100. At least a portion of the peripheral wall 104 is heated to a reaction temperature to provide heat for the reaction of the precursor. The peripheral wall 104 includes a top wall 104A, a bottom wall 104B and side walls 104C. The internal space 106 of the elongated chamber may be further defined by an inner surface 107 of the top wall 104A or the inner top surface 107, an inner surface 109 of the bottom wall 104B or the inner bottom surface 109, and inner surfaces 111 of the side walls 104C or the inner side wall surfaces 111. The internal space 106 may be divided into several sections or temperature zones of the elongated chamber 100. Accordingly, the axis of the elongated chamber 100 and the inner space 106 defined by the inner surfaces 107, 109 and 111 is preferably parallel to the x-axis, the transverse axis is parallel to the y-axis, and the vertical axis is parallel to the z-axis. As will be described below, the peripheral wall 104 of the elongated chamber 100 may include one or more inner layers called liner or insert which may be made of high thermal conductivity materials.

The process direction ‘P’ is parallel to the axial direction of the elongated chamber 100. In this embodiment an axial length of the elongated chamber is greater than the width or transverse length of the elongated chamber 100. In a roll-to-roll process operation, the continuous workpiece 102 may be a sheet-shaped workpiece that enters the process space from the entrance opening 112A, advances though process space and leaves the process space from the exit opening 112B. In this embodiment, the front surface 103A of the continuous workpiece is exposed to the process space 106 and the process gas as the workpiece 102 is advanced therethrough while the back surface 103B is supported by the inner bottom surface 109 of the bottom wall 104B. The process gas, including at least one of a reactive gas such as selenium or a selenium-containing gas and an inert gas such as nitrogen, may be delivered into the process space 106 through the entrance opening 112A or a gas inlet adjacent to the entrance opening in the process direction ‘P’. An exhaust gas that is produced during the reaction may be removed through a gas exhaust located between the entrance opening 112A and the exit opening 112B. As mentioned above, the elongated chamber 100 and hence the internal process space 106 may be divided into several sections or zones including different heating and cooling characteristics to process the workpiece 102 as it is advanced therethrough. In one embodiment, the process space 106 of the elongated chamber may have a width in the range of 10-200 cm and a height in the range of 2-40 mm.

FIG. 3A shows an exemplary portion 100A of the elongated chamber 100 including a reduced contact surface region 120 to reduce or minimize the physical contact between the inner bottom surface 109 and the back surface 103B of the workpiece 102 during the process. The reduced contact surface region 120 may be in any one of the sections of the elongated chamber such as a heating section, cooling section and any isothermal heating sections. The reduced contact surface region 120 may include a plurality of protuberances 122 projecting upwardly from the inner bottom surface 109 of the bottom wall 104B. In this respect, the protuberances 122 may be a part of the bottom wall 104B or any liner layer, which will be described more fully below, placed over the bottom wall. The protuberances 122 may be three dimensional shapes such as hemispheres, balls, cylindrical surfaces, pyramids, cones or other geometrical or non-geometrical shapes having a reduced uppermost end 124 to significantly reduce the contact area between the workpiece 102 and the protuberance.

In one embodiment, the protuberances 122 may be formed as a part of the bottom wall using many manufacturing methods such as direct machining into the desired sections of the bottom wall material, such as silicon carbide, aluminum nitride, graphite, silicon nitride, aluminum oxide or using other known manufacturing methods to manufacture with the bottom wall material. When formed as the part of the bottom wall, both the protrusions and the bottom wall are made of the same material. However, a special surface coating layer including a low thermal conductive material, such as fused silica, and other amorphous ceramics, ceramoplastics, glass-ceramic, microporous ceramics, and the like, may be coated on such manufactured protuberances to make the protrusions less thermally conductive than the bottom wall material which may be a high thermal conductivity material. Alternatively, the protuberances may be manufactured as separate pieces which may include the same material as the bottom wall material or a different material from the bottom wall material. When separate pieces such as rods or balls are used, the protuberances may be made of low thermal conductivity materials, such as fused silica, whereas the bottom wall is made of high thermal conductivity material such as silicon carbide or aluminum nitride. The separate protuberance pieces may be attached or fastened to the bottom wall using many methods. For example, they may be inserted into recesses that are preformed in the desired sections of the bottom wall 104B. Best results may be obtained if such protuberances are thermal insulators and resistant to high temperature corrosion and mechanical wear applied by the workpiece.

The height of the protuberances above the inner bottom surface 109 of the bottom wall 104B can be quite small, for example on the order of a few millimeters, preferably 0.5-5 mm, and more preferably 1-3 mm. Such small protuberances elevate the workpiece 102 off the inner surface of the bottom wall 104A while minimizing the volume under the workpiece 102. In addition, the protuberances 122 may disrupt viscous process gas flow under the workpiece 102 and cause a series of small and uniform outward process gas flows which have a less disruptive effect than the sort of large non-uniform and multidirectional gas flows described above in the background section. The protuberances may be distributed as a pattern along rows extending along the transverse axis and/or columns extending along the axis on the bottom wall 104B. In this respect if the protuberances are laid out as rows, each row may include a single piece protuberance such as cylindrical rods, triangular rods, rectangular rods and the like. Each row may be made of multiple pieces of protuberances including more than one protuberance such as rows of hemispherical, pyramidal, conical, cylindrical or the like protuberances. A preferred distance between the uppermost ends of the protuberances along the axis and the transverse axis may be in the range of 25-50 mm.

An exemplary width for the protuberances may be less than 20 mm. The uppermost ends 124 of the protuberances may be coplanar. The protuberances may have the same height, for example, in the range of 1-10 mm. However, some of them may be made shorter to reduce the incidents of physical contact with the reduced contact surface region while still assisting to disrupt non-uniform gas flow under the workpiece. The protuberances 122 may preferably made of materials that are resistant to mechanical wear, especially, at high temperatures such as silicon carbide and aluminum nitride. When such hard materials are used to form the bottom wall of the elongated chamber, the protuberances or ribs may be formed in the bottom wall by directly machining the bottom wall. Exemplary materials for protuberances may be fused silica, silicon carbide, aluminum nitride, glass ceramics and zirconia.

In one implementation, the reduced contact surface comprise a plurality of parallel spaced apart ridges or rods that are approximately 25-50 mm apart and extend substantially across the width of the chamber. These ridges define a contact surface that is approximately between 0.25 mm wide to 1 mm wide in the direction of the process direction P that contact with the bottom surface 103B of the workpiece. In this implementation, the ridges inhibit gas flow in the direction parallel to the direction of process direction P. There may be 20-40 ridges per linear meter. In another implementation, the discrete protrusions, such as fixed balls, pyramids, cylinders etc., may define a contact surface of approximately 1 mm² that contacts the bottom surface of the workpiece 102 and the protrusions are formed to a density of about 2500 protrusions per square m.

To aid transfer of heat to the process space, it is best to construct or line the elongated chamber of the reactors with materials which have high thermal conductivity such as silicon carbide, graphite, aluminum nitride, and the like high thermal conductivity ceramics or dielectric materials. However such materials can inhibit the maintenance of thermal gradients or profiles configured for the reactor. Because when the entire elongated chamber walls are constricted or lined with such materials and a specific section is heated to a temperature that is different than the neighboring sections have, the applied heat may transfer not only towards this heated zone, but also towards the neighboring zones maintained at different temperature thereby disturbing the desired thermal gradient. An example may be a reaction zone surrounded by heating and cooling zones which need to be maintained at temperatures other than the hotter reaction zones. Furthermore, as it is advanced through the reactor, the heated workpiece or web also transports heat in the direction of motion or the process direction ‘P’. Such transportation of heat with the workpiece may also make precise temperature control or implementing a temperature profile within the reactor more difficult, especially when the process space is heated through the heat transferred from the walls of the reactor chamber. As will be described more fully below, one solution to such problems is to construct the walls of the elongated chamber or inserts (liners) out of materials which have high thermal conductivity but disrupt the liner walls on a periodic basis with sections that have low thermal conductivity such as fused silica and other amorphous ceramics, ceramoplastics, glass-ceramic board, microporous, ceramics, and the like.

FIG. 3B shows, in a partial side view, an embodiment of the elongated chamber 100 having a high thermal conductivity lining 130 or insert placed into the process space 160 defined by the peripheral wall 104 of the elongated chamber 100. The insert 130 may be a lining layer or inner enclosure inserted into the process space in a manner that the insert at least partially contacts the inner surface of the peripheral wall 104.

As also shown in FIG. 3C, in a front cross sectional view, the insert 130 may include a top plate 130A or top wall, a bottom plate 130B or bottom wall and side plates 130C or side walls defining an inner space 160B to receive the workpiece 102 for processing. In this configuration, the peripheral wall 104 of the elongated chamber 100 is heated according to a desired thermal profile. Such regulated heat transfers from the peripheral wall 106 to the insert 130 to apply the desired heat pattern to the workpiece advancing through the inner space 160A of the insert 130. The inner space 160A of the insert may be dimensioned to have the same width and length ranges given in the above embodiment by appropriately dimensioning the insert and the peripheral walls 104. In this embodiment, similar to the previous embodiment, at least a portion of the bottom plate 130B of the insert may have the reduced contact surface region 120A including protuberances 122A with uppermost ends 124A.

In FIGS. 3B-3C, the protuberances are exemplary hemispherical protuberances. However the protuberances may be shaped as rods or ribs extending along the width of the bottom plate 130B. In an alternative embodiment, the insert 130 may be comprised only of the bottom plate 130B including the reduced contact surface region 120A, i.e., no top plate and side plates. In either embodiment, the protuberances 120A of the insert 130 are designed, dimensioned and manufactured using the same principles described in the previous embodiment for the protuberances 120.

In one embodiment, the protuberances 122A of the insert may be formed as a part of the bottom plate 130B using, for instance, direct machining into the desired sections of the bottom plate material, such as silicon carbide or aluminum nitride, or using other known manufacturing methods to manufacture with the bottom plate material. When formed as the part of the bottom plate, both the protrusions 122A and the bottom plate 130B are made of the same material. However, a special surface coating layer including a low thermal conductivity material, such as fused silica, amorphous ceramics and flame sprayed ceramics such as alumina, zirconia, yttria-stabilized zirconia, and the like may be coated on such manufactured protuberances to make protrusions less thermally conductive than the bottom plate material which may be a high thermal conductive material. Alternatively, the protuberances 122A may be manufactured as separate pieces which may include the same material as the bottom plate material or a different material from the bottom wall material. When separate pieces such as rods or balls are used, the protuberances may be made of low thermal conductivity materials, such as fused silica, whereas the bottom plate of the insert is made of high thermal conductivity materials such as silicon carbide or aluminum nitride. The separate protuberance pieces may be attached or fastened to the bottom plate 130B using many methods. For example, they may be inserted into recesses that are preformed in the desired sections of the bottom plate 130B or may be glued to the bottom plate using appropriate adhesives.

FIGS. 4A-4B show another embodiment of the elongated chamber 100 having an insert 140 with high thermal conductivity regions 142 and low thermal conductivity regions 144. In this embodiment, the low thermal conductivity regions 144 of the insert 140 physically and thermally separate the high thermal conductivity regions from one another. Each high thermal conductivity region 142 may form the inner walls of the at least one of a cooling, heating or isothermal heating section of the elongated chamber 100. The low thermal conductivity regions located between the high thermal conductivity regions 142 facilitate independent temperature control of the thermally isolated heating or cooling zones. Heat from the peripheral wall 104 or the bottom wall 104B or the top wall 104A of the elongated chamber 100 may transfer to adjacent high conductivity region 142 vertically and heat or cool the workpiece 102. The low thermal conductivity regions 144 surrounding the high conductivity regions 142 minimize lateral heat transfer between the neighboring high thermal conductivity regions. As shown in FIG. 4B, high thermal conductivity regions of a bottom plate 140B of the insert 140 may include reduced contact surface regions 120B including protuberances 122B with uppermost ends 124B. In this embodiment, the reduced contact surface regions 120B are manufactured and dimensioned the same as the reduced contact surface regions 120A described above.

FIG. 5A shows an exemplary roll-to-roll reactor 200 including an elongated chamber 202 with an exemplary insert 204 defining a process space 206 to process a workpiece 102. The insert 204 is surrounded by the peripheral wall 205 of the elongated chamber 202, wherein the top wall 205A, bottom wall 205B and side walls (not shown) are constructed in the manner described in the above embodiments. Of course, the use of insert 204 is exemplary, the features described below can be directly formed in the peripheral wall of the elongated chamber 202, as described in the above embodiments. Various features of an elongated chamber of the present invention are described above for the elongated chamber 100, and such features are not included in FIG. 5A for clarity. In a roll-to-roll process operation, the continuous workpiece 102, which is a sheet-shaped workpiece, is unwound from a supply spool 116A as a fresh or unprocessed workpiece, advanced through the process space 206 and picked up and wound around a receiving spool 116B as a reacted or processed workpiece. In this embodiment, the top surface 103A of the continuous workpiece 102 is exposed to the process space 206 and its environment as the workpiece is advanced therethrough while the back surface 103B is supported by a bottom plate 204B of the insert 204. As described, the top surface 103A includes a precursor layer such as a CIGS precursor layer to be processed or reacted and the back surface 103B is the back surface of a continuous substrate such as a stainless steel foil. A moving mechanism (not shown) tensions, advances, and supports the workpiece as the workpiece is advanced through the elongated chamber 202.

FIG. 5B shows a thermal profile graph 300 of the elongated chamber 202 shown in FIG. 5A. In this embodiment, the elongated chamber 202 may include a first low temperature isothermal zone 1Z or first zone A, a heating zone 2Z or second zone B, a high temperature isothermal zone 3Z or third zone C, a cooling zone 4Z or fourth zone D, and a second low temperature isothermal zone 5Z or fifth zone E. The high temperature isothermal zone 3Z may be heated by heating elements 218 located in the top wall 205A and the bottom wall 205B of the elongated chamber 202. For CIGS precursor selenization, the high temperature isothermal zone 3Z may be heated to a temperature range of 250-750° C. Optionally, the heating zone 2Z and the cooling zone 4Z may also have heating elements 218. Segments A, B, C, D and E of the thermal profile graph 300 shown in FIG. 5B correspond to the zones 1Z-5Z of the elongated chamber 200 shown in FIG. 5A respectively. The elongated chamber 202 may include additional heating or cooling zones adjacent the existing heating or cooling zones. The first and second low temperature isothermal zones 1Z and 5Z may be water cooled.

Referring to FIG. 5A, the insert 204 may include a number of high thermal conductivity regions 210 and separated by low thermal conductivity regions 212, which are also described above with respect to FIGS. 4A-4B. In this embodiment, the heating zone 2Z (heat ramp-up zone) and the cooling zone 4Z (heat ramp-down zone) include the high thermal conductivity regions 210. The low thermal conductivity regions 212 placed between the beginning and the end of the zones 2Z and 4Z separate them from the adjacent high thermal conductivity regions so as to better thermally control the heating and cooling zones. Heat from the peripheral wall 205 of the elongated chamber 202 transfers to the high thermal conductivity regions surrounding the zones 2Z and 4Z, and due to the low thermal conductivity regions limiting them, heat transfer from or to the adjacent zones is minimized, thereby allowing a better control of temperature in such zones. As seen in the graph 300, segments of the thermal profile corresponding to the low thermal conductivity regions 212 of the insert cause sharp temperature changes. Further, the bottom plate portions of the heating and cooling zones 2Z and 4Z are equipped with reduced contact surface regions 120C including protuberances 122C. The protuberances 120C are formed in the high thermal conductivity regions 210. In this embodiment, the protuberances 122C may be embedded rods, such as cylindrical rods, extending transverse direction of the elongated chamber. As described above the protrusions may be made of materials resistant to mechanical wear. This, in turn, may allow the bottom walls of the elongated chamber or the bottom insert plates to be manufactured from materials that are vulnerable to mechanical wear such as graphite because of the lack of direct contact of the workpiece with the inner surfaces of the bottom wall to which the protuberances are attached. It will be appreciated that the temperature of the process gas is changing in zones 2Z and 4Z which can create turbulence in the gas flow. It may be desirable to reduce that turbulence by changing the volume profile of the zones 2Z and 4Z in the manner that is described in the Assignee's co-pending patent application entitled THERMAL REACTORS WITH IMPROVED GAS FLOW CHARACTERISTICS, U.S. patent application Ser. No. 13/100,094, filed May 3, 2011.

The embodiment shown in FIG. 5A may be used for designing the interior of a thermal reactor, tunnel oven or muffle furnace for roll-to-roll (continuous workpiece or web based) thermal processing. In one embodiment, this reactor design may be used for selenization of CIGS precursors to form CIGS absorbers for photovoltaic devices. However, other potential uses may also be envisioned, such as CVD or oxidation furnaces for thin film processing of solar cells, x-ray detectors and semiconductor devices. Although this invention may be used with reactors of furnaces employing other or less direct methods of heating such as radiation or heating jackets, the elongated chamber walls of the present invention include embedded heating elements 218 which may be resistance heaters.

Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art. Hence, the scope of the present invention should not be limited to the foregoing description but should be defined by the appended claims. 

1. A roll-to-roll thermal reactor to heat and react a precursor material disposed over a continuous workpiece to form a solar cell absorber, the reactor comprising: an elongated process enclosure defined by a peripheral wall including a top wall, side walls and a bottom wall, wherein the continuous workpiece enters the elongated process enclosure from an entrance opening, advances through the elongated process chamber while contacting the bottom wall, and exits from an exit opening; and at least one reduced contact surface region formed in an inner surface of the bottom wall so that the physical contact between the workpiece and the bottom wall is reduced, wherein the reduced contact surface region includes a plurality of fixed protrusions projecting upwardly from the inner surface of the bottom wall and wherein the workpiece touches topmost ends of the plurality of fixed protrusions.
 2. The reactor of claim 1, wherein the elongated process enclosure includes at least one heating section to heat the workpiece to a reaction temperature, at least one reaction section to react the precursor material of the workpiece at the reaction temperature and at least one cooling section to cool the workpiece with the reacted precursor.
 3. The reactor of claim 2, wherein the reduced contact surface region is formed in at least one of the heating and cooling sections.
 4. The reactor of claim 3, wherein the shape of the fixed protrusions includes at least one of a hemispherical shape, a cylindrical shape, a conical shape, pyramidal shape and rectangular shape.
 5. The reactor of claim 4, wherein the height of the fixed protrusions is in the range of 0.5-15 mm and the distance between the topmost ends is in the range of 25-500 mm.
 6. The reactor of claim 5, wherein the fixed protrusions are cylindrical rods extending along the width of the bottom wall.
 7. The reactor of claim 6, wherein the height of the cylindrical rods is in the range of 0.5-15 mm and wherein the distance between top most end of the cylindrical rods is in the range of 25-500 mm.
 8. The reactor of claim 3 wherein the heating, reaction and cooling sections of the elongated process enclosure are formed of a layer of a first material, and wherein each section is separated from one another by a layer of a second material, and wherein the thermal conductivity of the second material is less than the thermal conductivity of the first material.
 9. The reactor of claim 8 wherein thermal conductivity of the first material is greater than 5 W/m-K, and the thermal conductivity of the second material is less than 2 W/m-K.
 10. The reactor of claim 8, wherein the first material includes one of silicon carbide, aluminum nitride and graphite, and the second material includes one of fused silica, amorphous ceramics, ceramoplastics, glass-ceramic, microporous ceramics.
 11. The reactor of claim 10, wherein the plurality of fixed protrusions are made of the first material.
 12. The reactor of claim 11, wherein the plurality of fixed protrusions are made of the second material.
 13. The reactor of claim 1, wherein the bottom wall is made of a first material and the plurality of fixed protrusions are made of a second material, and wherein the thermal conductivity of the first material is greater than the second material.
 14. The reactor of claim 13, wherein the bottom wall and the plurality of fixed protrusions are made of a first material.
 15. The reactor of claim 14, wherein the first material includes one of silicon carbide, aluminum nitride, alumina, zirconia, magnesia-stabilized zirconia, yttria-stabilized zirconia and graphite, and the second material includes one of fused silica, and other amorphous ceramics, ceramoplastics, glass-ceramic, microporous ceramics.
 16. The reactor of claim 1, wherein the elongated process enclosure is an insert placed in a space defined by a peripheral reactor wall, and wherein the peripheral reactor wall include heating elements.
 17. A solar cell processing system comprising: a continuous workpiece containing a precursor material used to form a solar cell absorber; a reactor having a inlet and an outlet and a central chamber disposed between the inlet and the outlet, wherein the continuous workpiece is provided into the inlet of the reactor and extends through the central chamber and exits via the outlet wherein a portion of the reactor includes a reduced contact surface where the bottom surface of the continuous workpiece is intermittently supported by the reduced contact surface; a heating system that heats the reactor; and a gas supply system that provides a reactive gas into the reactor so that the combination of the reactive gas and the heat from the heating system transforms the precursor material on the continuous workpiece into the solar cell absorber.
 18. The system of claim 17, wherein gas is introduced into the reactor via the gas supply system adjacent the inlet and the exhaust gas is removed through an exhaust located between the inlet and outlet.
 19. The system of claim 18, wherein the reactor has an expansion region interposed between the inlet and the central chamber where the gas expands as a result of the heating system heating the central chamber and a contraction region interposed between the central chamber and the outlet where the gas contacts as a result of the gas cooling after leaving the central chamber.
 20. The system of claim 19, wherein the reduced contact surface is located in the gas expansion region.
 21. The system of claim 20, wherein the reduced contact surface is also located in the gas contraction region.
 22. The system of claim 20, wherein the reduced contact surface comprises a plurality of fixed protrusions that contact the bottom side of the continuous workpiece at intermittent locations.
 23. The system of claim 22, wherein the plurality of fixed protrusions comprise ridges that extend substantially across the width of the continuous workpiece.
 24. The system of claim 23, wherein the ridges are approximately 25-500 mm apart.
 25. The system of claim 22, wherein the plurality of fixed protrusions comprise discrete protrusions that are spaced apart from each other having an average density of 100 10,000 protrusion per m².
 26. The system of claim 22, wherein the fixed protrusions are formed of a low thermal conductivity material.
 27. The system of claim 26, wherein the fixed protrusions are formed of fused silica.
 28. The system of claim 21, wherein the reduced contact area is formed on an insert that is positioned in the reactor.
 29. The system of claim 28, wherein the insert layer is a continuous layer including high thermal conductivity portions defining at least one heating section, at least one reaction section and at least one cooling section and low thermal conductivity portions placed between the sections.
 30. A solar cell processing system comprising: a continuous workpiece containing a precursor material used to form a solar cell absorber; a reactor having a inlet and an outlet and a central chamber disposed between the inlet and the outlet, wherein the continuous workpiece is provided into the inlet of the reactor and extends through the central chamber and exits via the outlet wherein the reactor includes heating, reaction and cooling sections and wherein the heating, reaction and cooling sections of the reactor are formed of a layer of a first material, and wherein each section is separated from one another by a layer of a second material, and wherein the thermal conductivity of the second material is less than the thermal conductivity of the first material; a heating system that heats the reactor; and a gas supply system that provides a reactive gas into the reactor so that the combination of the reactive gas and the heat from the heating system transforms the precursor material on the continuous workpiece into the solar cell absorber.
 31. The system of claim 30, wherein a portion of the reactor includes a reduced contact surface where the bottom surface of the continuous workpiece is intermittently supported by the reduced contact surface.
 32. The system of claim 31, wherein the reduced contact surface is located in the heating section.
 33. The system of claim 32, wherein the reduced contact surface is also located in the cooling section.
 34. The system of claim 32, wherein the reduced contact surface comprises a plurality of fixed protrusions that contact the bottom side of the continuous workpiece at intermittent locations.
 35. The system of claim 34, wherein the plurality of fixed protrusions comprise ridges that extend substantially across the width of the continuous workpiece.
 36. The reactor of claim 30 wherein thermal conductivity of the first material is greater than 5 W/m-K, and the thermal conductivity of the second material is less than 2 W/m-K.
 37. The reactor of claim 30, wherein the first material includes one of silicon carbide, aluminum nitride and graphite, and the second material includes one of fused silica, amorphous ceramics, ceramoplastics, glass-ceramic, microporous ceramics. 